Aircraft landing gear are critical components that are highly stressed and subject to adverse environmental conditions in use. Steel alloys such as AISI 4340 and the 300M alloy have long been used to make landing gear for aircraft because those alloys can be quenched and tempered to provide very high strength (ultimate tensile strength of at least 280 ksi) in combination with fracture toughness (KIc) of at least 50 ksi√in. However, neither of those alloys provides effective corrosion resistance. Therefore, it has been necessary to plate the landing gear components with a corrosion resistant metal such as cadmium. Cadmium is a highly toxic, carcinogenic material and its use has presented significant environmental risks in the manufacture and maintenance of aircraft landing gear and other components made from these alloys.
A known alloy that is sold under the registered trademark FERRIUM S53 was developed to provide a combination of strength and toughness similar to that provided by the 4340 and 300M alloys and to also provide corrosion resistance. The FERRIUM S53 alloy was designed to overcome the problems associated with using cadmium plating to provide adequate corrosion resistance in aircraft landing gear made from either the 4340 alloy or the 300M alloy. However, the FERRIUM S53 alloy includes a significant addition of cobalt which is a rare and thus, expensive element. In order to avoid the much higher cost of using the FERRIUM S53 for the landing gear application, attempts have been made to develop a quench and temper steel alloy that provides the strength, toughness, and corrosion resistance attributed to the FERRIUM S53 alloy, but without the addition of costly cobalt.
Cobalt-free martensitic steel alloys that can be quenched and tempered to provide strength and toughness comparable to the FERRIUM S53 alloy and which also provide corrosion resistance are described in U.S. Pat. No. 8,071,017 and in U.S. Pat. No. 8,361,247. However, it has been found that the corrosion resistance provided by those steels leaves something to be desired. Enhanced corrosion resistance is especially important for aircraft landing gear because they are exposed to many different types of corrosive environments, some of which are more aggressive than others at causing corrosion in steel. Accordingly, there is a need for a steel alloy that provides the very high strength and toughness needed for the landing gear application, that provides better corrosion resistance than the known corrosion resistant quench and temper steels, and that can be produced at a discount in price relative to steels that contain a substantial amount of cobalt.
Furthermore, known martensitic steel alloys are generally melted via conventional means, including vacuum induction melt (VIM), and VIM/vacuum arc remelting (VAR). The known alloy is then cast into ingot form, and processed either through rolling or forging to obtain the final desired product, either billet or bar. However, there is a desire in the aerospace industry for near net shape processing, so that parts can be manufactured with much less machining and less waste of material compared to conventional processing such as machining from bar or rough forged billet.